Recent technology advances have led to a significant interest in CMOS active pixel sensors (APS) as replacements for other image sensors. Such sensors typically have a photodiode for generating a pixel image signal. The photodiode is typically reset to a known voltage before image change integration by the photodiode. Both the photodiode reset signal and the photodiode image signal are sampled and read out and subtracted (reset signal−image signal) to produce the actual pixel integrated charge signal.
One observed artifact with such sensors is the presence of dark images which occurs when pixels oversaturate due to very bright images. When the pixel image signal saturates, the signal level becomes very close to ground. As the photocurrent produced by the photodiode continues to increase at pixel image signal saturation, the photodiode voltage will start to reduce during the time of the pixel reset and reset signal level sampling. This will cause the sampled reset signal level to be reduced. With increasing photocurrent, the reset signal level will continue to drop. Since the image signal level is saturated near ground and the reset signal level is dropping, the apparent integrated signal (reset signal level−image signal level ) will actually decrease and the resulting digital image will have a dark spot in the oversaturated region. This can create a very disturbing effect in the final image, especially when an object like the sun is captured, giving an eclipse-like image. There is no clear method for identifying this condition after an image signal has been digitized.
One proposal to correct for this effect is to flag saturated pixels in the pixel sampling circuitry with a comparator so that remedial action can be taken by downstream circuits. Thus, a comparator compares each pixel's image signal level prior to pixel reset. All pixels with an image signal level below a certain voltage are flagged as saturated and a digital “saturated” value may be substituted in place of the pixel output (reset signal−image signal) digitized value by on-chip timing and control logic. Since sampling typically occurs on a column-wise basis, each column circuit needs its own comparator. This comparator will also need a voltage reference, corresponding to a threshold level just enough above ground to reliably flag saturated pixels without significantly reducing the maximum signal swing out of the pixels. This is often a value of less than 100 mV. Unfortunately, reliable generation of this threshold voltage and providing a voltage generating circuit to produce this voltage takes power and additional chip area. Accordingly, a simpler technique which consumes less power is desired.